Cellulose-reactive sizing agents, such as those based on alkyl ketene dimer (AKD) and alkenyl succinic anhydride (ASA), are widely used in papermaking at neutral or slightly alkaline stock pH's in order to give paper and paper board some degree of resistance to wetting and penetration by aqueous liquids. Paper sizes based on cellulose-reactive sizing agents are generally provided in the form of dispersions containing an aqueous phase and finely divided particles or droplets of the sizing agent dispersed therein. The dispersions are usually prepared with the aid of a dispersant system consisting of an anionic compound, e.g. sodium lignosulfonate, in combination with a high molecular weight cationic or amphoteric polymer, e.g. cationic starch, polyamine, polyamideamine or a vinyl addition polymer. Depending on the overall charge of the compounds of the dispersant system, the size dispersions will be cationic or anionic in nature.
Cellulose-reactive sizing agents generally provide good sizing with low dosages of the sizing agent. However, it has been experienced that the efficiency of conventional cellulose-reactive sizing agents is deteriorated when they are used with stocks having a high cationic demand and containing a substantial amount of lipophilic wood extractives, such as, for example, resin acids, fatty acids, fatty esters, triglycerides, etc. Due to the anionic character of lipophilic substances containing carboxylate or carboxylic acid groups, stocks containing a substantial amount of lipophilic extractives usually have a rather high cationic demand, it has been found that the lipophilic substances can be detrimental to the adsorption of sizing agents onto the fibers which may cause the poor sizing results. In order to improve sizing with such stocks the papermaker has had to increase the dosage of sizing agent, which of course is less favourable economically and can increase the accumulation of sizing agent in the white water recirculating in the papermaking process. These problems are even more pronounced in paper mills where white water is extensively recirculated with the introduction of only low amounts of fresh water into the process, thereby further increasing the cationic demand and the accumulation of lipophilic extractives and non-retained sizing agent in the white water and the stock to be dewatered.
It is accordingly an object of this invention to provide improved dispersions of cellulose-reactive sizing agent and processes resulting in improved sizing where dispersions of cellulose-reactive sizing agent are used with cellulosic stocks having a high cationic demand and/or a high content of lipophilic extractives and/or processes with extensive white water recirculation.